Nexus One display and subpixel pattern

According to Wikipedia, this is what makes an AMOLED display different from standard LCD:

It does not need a backlight, the pixels emit light themselves. This allows for thinner display.

Only the turned-on pixels consume power. In average AMOLED uses less power than LCD. The power usage is dependent on the color displayed — a black screen uses much less power than a white screen.

The contrast is very good. The black is very deep (as there’s no backlight leakage).

The viewable angle is very large; this is often a problem with LCD, particularly TN (twisted-nematic), which have small viewing angles.

OLED has better reponse rate than LCD.

The lifetime of OLED is shorter than LCD. Only the period a pixel is lightened (turned-on) counts towards the lifetime. The blue subpixels have the shortest lifetime.

Glossing over the technical details, the result is that the Nexus One has a *great* display. It is the highest quality display I’ve ever seen (but I can’t say how it compares to the Droid display as I have not seen a Droid phone yet). The Nexus display is large and high-density, is very crisp, the colors are very saturated, and the black is indeed very deep. WVGA, 800×480 is quite a high resolution for a display of this size (a more typical resolution for this size would be HVGA, 480×320 as on the G1).

After getting used to the Nexus display, when you look back to your previous phone’s display you’ll be shocked by how low-quality the old LCD seems now.

The AMOLED display on Nexus has one more surprise on hold: On Nexus, each pixel is composed of only two subpixels instead of the usual three (Red, Green, Blue) subpixels of an LCD. The following picture should help understand the subpixel pattern on a Nexus: (source)

So every pixel contains a green subpixel, and alternating a red or a blue subpixel. A pixel has either red or blue subpixels but not both.

Why is it done this way? This technique allows for a larger physical area for the blue and red subpixels, thus increasing their lifetime. It also allows to implement the high pixel count (800×480) by using only two thirds of the subpixels that would normally be needed (2subpixels/pixel instead of 3subpixels/pixel).

What is the impact of this unusual subpixel pattern? Natural images (such as pictures, movies etc) are very well reproduced and likely the subpixel impact is not perceivable. Synthetic images that contain pixel-aligned thin and saturated lines (red or blue) allow to discern the “alternating” subpixel pattern. But as the display is very high density and the pixels are very small, most likely you would never become aware of the subpixel pattern if not looking explicitly for it.

What is the impact of AMOLED for application developers:

First of all, you may not care at all about AMOLED vs LCD, and everything will work just fine.

You can use darker colors to save power. You may prefer white-text-on-black-background to black-on-white-background, as the dominant black color uses less power on AMOLED. (the dominant color makes no power difference on classical LCD).

You may prefer using darker colors to increase the display lifetime. You may prefer avoiding displaying intense blue (as it has the lowest lifetime) for long periods of time.

If you want to draw highest-resolution thin lines, they are best rendered in green — because the green subpixels have double the resolution of red and blue subpixels.

It seems the choice of subpixel pattern is also related to human eye physiology, as the human eye is more sensible to green than to blue, and keeping green at full resolution gives rich image information to the eye.

I have a Droid and iPhone 3Gs sitting next to each other and while the Droid still has a more conventional TFT LCD screen, it shares the high-res WVGA size (technically it’s even more hi-res than the Nexus One since it’s 854×480 in the same 3.7″). It makes a huge difference, which makes sense when you realize there are 2.5x the pixels (409,920 vs. 153,600). The difference is most noticeable in the browser where small text loves to lurk.

While the human eye may be more sensible to green than to blue, I’m sure I’ve read somewhere that it’s even more sensible to red. I guess this may be due to red being a danger colour for humans. You haven’t mentioned how the lifetime of a red pixel compares to a green or blue pixel, but I guess it must be significantly shorter than green, otherwise it would make more sense to have more blue and green pixels and less red pixels.

This is not true. Darkening a traditional LCD requires twisting the nematic crystal, which then quickly decharge and untwist. Recharging the black pixels over and over again can consume half a watt, or 10% of my laptop’s total power.

*PLEASE* don’t use acronyms like “HVGA” or “WVGA”. They serve *no* useful purpose whatever… they are harder to understand than pixel counts and only serve to unnecessarily jargonize screen sizes, and they aren’t significantly shorter than just putting in the literal pixel dimensions.

Indeed, you found it necessary to include both, presumably because you agree that HVGA and WVGA are hard to understand. You would have had less typing to do, and improved readability in the result, *without* these unnecessary acronyms.

I wouldn’t say this is hardly noticeable; this has bothered me since the day I received mine. Photos look fine, but text and vector graphics have a horrid red sawtooth pattern on one side, and a green fringe on the other. It looks like botched subpixel antiailising. I’m debating returning my N1 and getting a Droid (or just waiting to see what comes next) as a result of this. I could learn to live with this, but it’s hardly desirable.

Come to think, this may have worked better had the red-blue ordering not changed every other row. I didn’t notice the green fringe on the right side of light areas until I went looking for it, but I noticed the red-blue-red-blue sawtooth artifact on the left within a few minutes of using the display. Had there just been a red or blue fringe on the left, I might not have noticed.

Nope. Most sensitive to green (around the 530 – 550 nm mark). Considerably less sensitive to red and green by about 1-2 orders of magnitude (See: http://www.yorku.ca/eye/lambdas.htm and look at the dotted line; that is your colour vision. The scale is logarithmic, so a plus or minus one on the relative sensitivity axis is a difference of ten times the sensitivity). So the relative pixel size makes sense.

>” I guess this may be due to red being a danger colour for humans.”

I’m sure I read somewhere that this is purely cultural – that in some cultures red stands for purity, joy, good luck, happiness, prosperity, etc., but I’m not sure. I will have to Google it.

The reason to have that kind of a sub pixel level might be to get good luminance.
Luminance is derived as 0.2126 R + 0.7152 G + 0.0722 B (brightness of the light coming from display), and to reproduce bright white either they have to change the intensity of Red and Blue (how its done in CRT display’s) or have less area for Red and Green compared to Green. In LCD its not a problem because they have a Bright white light as back light and to show white color on screen they just make that pixel transparent.

I think it is hard to tell much difference when holding a AMOLED vs a LCD at regular viewing distance. Also, I would like to see how much power you could save by using darker colors, it makes sense but just curious how much of a difference. Thanks @Marcello for posting the close up pictures.

AMOLED screens rock! I search using Black Google Mobile at http://bGoog.com to get a longer battery life on my AMOLED phone and to reduce my data usage. I definitely have noticed a longer battery life since using only black backgrounds on my phone.

Your subpixel comparison is misrepresentative of the scale of the pixel sizes of the two displays. Although the PenTile display depiction does represent 4 pixels horizontally, it represents only 2 pixels vertically (2×2 vs. 3×1). To be fair, you need to double the vertical subpixel “density” on the PenTile depiction so that there are 4 rows of pixels instead of just two.

AMOLED screens are good for people like me who surf the web a lot. On my mobile I even use the Black Google Blackl at http://www.blackl.com to take advantage of the energy savings that this screens can achieve when displaying a Dark Google background.

Wow, I wonder what kind of screen the new iPad 3 has. I know its supposed to be the most amazing screen that has ever been on a Tablet or SmartPhone. I currently have an iPhone 4S and the screen looks amazing. This is a pretty interesting article breaking down the different pixel ratios, thanks.

[...] cope with the problem, Samsung has created the PenTile Matrix, a process which involves intertwining bigger blue and red LEDs in a display, thus resulting in better life, as bigger LEDs last longer, but diminishing the number of actual [...]